Synthesis of neat norbornene oligomers (primarily dimers) via homogeneous catalysis is described in the technical literature. While it would be desirable to convert norbornene into a mixture of dimers and trimers useful as a high-energy, liquid fuel, no reference of which the Applicants are aware discloses either a commcerially useful process for upgrading norbornene to a suitable mixture of norbornene dimers and trimers over a heterogeneous catalyst. Moreover, no reference of which the Applicants are aware teaches a mixture of norbornene dimers and trimers having properties appropriate for a high energy-density liquid fuel.
U.S. Pat. No. 4,416,710 to Anderson discloses missile fuel compositions using polynorbornene as a binder for liquid and solid materials.
U.S. Pat. No. 4,190,611 to Lyons et al. describes preparation of high-energy fuels by co-oligomerization of norbornene and norbornadiene using a complex three-component homogeneous catalyst (cobaltic or cobaltous acetylacetonate, 1,2-bisdiphenylphosphino ethane and an alkyl aluminum chloride).
A brief summary of several pertinent literature references describing oligomerization of norbornene with homogeneous catalysts is given below.
Photodimerization of norbornene is described by Salomen and Kochi (Salomen, R. G. & Kochi, J. K. Tetrahedron Letters No. 27, 2529, 1973) and by Arnold et al. (Arnold, D. R., Trecker, D. J., and Whipple, E. B., JACS, 2596, 1965). Salomen and Kochi achieved an 88% yield of almost pure exo-trans-exo norbornene dimer by irradiating a solution of norbornene in THF containing a copper triflate catalyst for six days. The product, however, was a solid with a melting point of +63.5.degree.-64.0.degree. C. and, thus, not suitable as a high-energy fuel.
Arnold et al. achieved a 26% yield of almost pure exo-trans-exo norbornene dimer by irradiating norbornene in an anhydrous ether solvent using cuprous bromide catalyst for 149 hours. The isolated exo-trans-exo norbornene dimer was a solid with a melting point of +64.degree.-65.degree. C. and, thus, was also not suitable as a high-energy fuel.
Arnold et al. also achieved a 23% yield of primarily endo-trans-exo norbornene dimer by irradiating norbornene in a benzene solvent for 192 hours. The isolated dimer product was also a solid having a melting point of +38.5.degree.-39.5.degree. C., and, thus, not suitable as a high-energy fuel.
Laverty et al. (Laverty, D. T., McKervey, M. A., Rooney, J. J., and Stewert, A., JCSCC 1368, 1976) produced a mixture of four different norbornene dimers (trans-anti, cis-anti, cis-syn, and trans-syn bi-2,2-norbornyldiene) by reacting norbornene in dry carbon sulfide or benzene solvents using soluble WCl.sub.6 or ReCl.sub.3 catalysts. The product also contained chlorinated derivatives which are incompatible with high-energy fuel applications.
Finally, Ito et al. (Ito, M., Ishii, Y., Hamanaka, S., and Ogawa, M., Sekiyu Gakkaishi (3), 246, 1986) achieved a 59.7% yield of almost exclusively a single norbornene dimer (trans-anti bi-2,2-norbornylidene) by reacting norbornene for six hours at 120.degree. C. in an ethanol solvent in a sealed glass tube using a nickel complex catalyst (Ni.sub.2 Cl.sub.2 (Ph.sub.3 P).sub.2 and NaBH.sub.4). No information is given on isolation and characterization of the properties of this dimer.
Thus it would be desirable to provide a process which selectively dimerizes norbornene over a heterogeneous catalyst to provide a product useful as a high density fuel.